That is because your coercion, of telling military-age males to die, is far more extreme when it involves 18-year olds losing the next 60 years of life. By contrast, you could be far more effective and obtain a far fairer/socially favorable result, by shifting to draft only the most suited for extreme risk operations, who sometimes, but not always, are the fresh faces who die young.
When the draft is started, it is in recognition that more, and specific, people are needed to prosecute the war (or other dangerous activity). Consider the groups of activities specifically involved in the prosecution of modern war (vs. e.g. the maintenance of the civil society, growing of food, etc.):
- Hitting a beach or similar shock-and-awe movement
- Transporting supplies, weapons, wounded, etc. in and out of the danger areas
- Sabotage operations and other irregular activities
- Specific intelligence gathering separate from combat operations, particularly by scouts and other humans
- The manufacture etc. of materiel
- The development of new weapons, countermeasures, etc.
- The allocation of resources to theaters, operational planning, etc.
Of these activities, far and away, the use of 18-year old males is most appropriate in any resource-constrained activity such as an operational push. It also is appropriate for certain special operations actions where extreme physical exertion is required to achieve the objective. But under those constraints, you only can make use of either:
- A limited number of men, because you have other resource constraints on e.g. their bullets, available armor, how many you can offload from ships, etc.
- The very strongest men for a limited number of special operations
Consequently, that labor pool is far better aligned to an all-volunteer capacity and the use of your best recruits; you quickly would reach exhaustion of other war-fighting prerequisites.
By contrast, if you need sheer mass of people to go over the line e.g. in support of armor, then it is far less cruel, to use the old and terminally ill as sandbags.
The transport tasks are a highly variable activity; but if dedicated staff are required, specific medical training is for instance indicated, and some modicum of strength. This is not something that requires young men; a better allocation is to use the middle aged and elderly.
The need for highly trained and physically elite special operators critically depends on the details of the operations in question; those that require a long trek with a lot of gear will need young men, but those that involve the placement of explosives, long periods of observation, mechanical support and extraction e.g. by helicopter, all can be serviced by the average person. In particular, extreme risk operations are best performed by the old and soon to die, because they can tolerate a much higher degree of danger in every one of their activities. Moreover, if they get wounded or captured, they’re going to die soon anyway, so the human cost of your irregular warfare greatly is reduced.
With human intelligence gathering, the military aged male is the worst person to draft for many reasons, but the most obvious being that if they blend in, they will be drafted by the enemy. You really want a mix of all types of people to line up to the available opportunities.
The manufacturing tasks also highly vary, but it’s all about skill: whoever has the skills you need for what you need to build, you bring on; age or other demographics are irrelevant.
Since the development of new weapons and countermeasures is specialized (but learnable), you are going to draft the smartest and cleverest (along with e.g. the most skilled machinists) people to handle these tasks.
Typically the planning operations are reserved for the pre-war general staff, so the draft is essentially irrelevant in these cases.